MULTICHANNEL RECORDING APPARATUS

Abstract: A body electrode monitor has a plurality of electrodes (6, 7, 9) for sensing potentials of points of attachment on a living body. A plurality of amplifiers (5, 8, 10) are provided each having two signal inputs and adapted for attachment to the body, for example, in a pouch carried by a belt. Data processing apparatus (14) is connected to the amplifiers. The sole connections between each amplifier and the electrodes are connections from the two signal inputs to two electrodes. Different amplifiers are connected to different combinations of electrodes. In this ambulatory monitoring arrangement, the whole arrangement is located within the environment of the subject's body and so no additional connection between the amplifiers and a common "earth" electrode on the living body is required.

Description: MULTICHANNEL RECORDING APPARATUS Many types of electrical signal can be picked up from the living body by applying pairs of electrodes to the body at points appropriate to detect the signals of interest. For example, a pair of electrodes applied to the head will pick up the potential difference between the points chosen due to the electroencephalogram signal from the brain. Again, a pair of electrodes applied to the thorax can pick up an electrocardiogram (ecg) signal from the heart. These signals are of relatively very small voltage compared to the interference voltage picked up on the body as a whole relative to earth due to the proximity of the subject to mains wiring, electrical apparatus, and other sources of electrical noise. It used to be the practice to apply a third or 'earth' electrode to the subject's body to reduce the effects of this interference and more recently this third electrode is still used to connect unearthed amplifier devices to the subject's body as a reference connection to try to equalise the interference voltage picked up on the subject's body and on the unearthed amplifier in apparatus separate from the subject's body, such as a trolley-mounted ecg recording machine, and 'bedside' ecg patient monitoring apparatus. In ambulatory monitoring the complete electronic system including amplifiers, recording or telemetering apparatus and/or automatic signal analysis apparatus is miniaturised and battery operated, and is adapted for wearing on the subject's body, for example in a pouch often mounted on a waist belt. The whole apparatus is small enough and close enough to the subject so that its environment is the subject's body and it exists within his 'electrical shadow' as it were. Thus the subject and his ambulatory monitoring apparatus are subject to the same electrical noise fields and the interference creates relatively little difference in potential between the ambulatory monitoring circuit chassis and the subject's body. The invention provides a body electrode monitor comprising a plurality of electrodes for sensing potentials at points of attachment on a living body, a plurality of amplifiers each having two signal inputs and adapted for attachment to the body, and data processing apparatus connected to the amplifiers, the sole connections between each amplifier and the electrodes being connections from the two signal inputs to two electrodes, different amplifiers being connected to different combinations of electrodes. The amplifiers preferably have a high, balanced input impedance to their common line. The adaptation of the amplifiers for support by the body means that the monitor is suitable for ambulatory monitoring, where the whole equipment is carried by the body. There is no need for a reference connection to an additional body electrode in addition to the connection of the two inputs already mentioned. The former practice of using a reference electrode and lead in ambulatory monitoring has been carried over from the conventional 'ground base' monitoring into ambulatory monitoring practice quite unnecessarily. In a preferred arrangement each electrode is connected to an input of a plurality of amplifiers, preferably by a single cable leading to a junction adjacent the amplifiers to keep the number of cables adjacent the electrodes (which might cause discomfort to the body) to a minimum. An example of the invention will now be described with reference to the accompanying drawings in which Figure 1 represents a prior art arrangement using three amplifiers, and Figure 2 represents an arrangement using three amplifiers according to the present invention. In Figure 1, each amplifier 11 has two inputs and each input is connected to its own body electrode. In addition, each amplifier is connected to an earth line which is connected to a further separate body electrode. Thus there are three connections between each amplifier and the electrodes. It will be seen that for three amplifiers seven body electrodes are required and in the general case, n amplifiers require (3n + 1) body electrodes. One amplifier is required for each body parameter being measured and so for a greater number of body parameters, the number 1 of body electrodes and their individual connecting cables becomes very large and is a cause of discomfort to the subject. In Figure 2, there are two basic differences over Figure 1. Firstly, there is no reference connection between each amplifier and a further reference body elecrrode. The other difference is that there are only two connections between each amplifier and the electrodes. The figure 2 arrangement contains the optional feature that each body electrode is connected to two inputs of differential amplifiers, one input from one amplifier, and another input from a different amplifier so that different combinations of only two electrodes are connected to different amplifiers. The amplifiers are differential amplifiers which have the advantage that the two active input connections of such an amplifier would have 'balanced', i.e. approximately equal, and relatively high impedances between the input terminals and the 'floating' common or chassis connection of the amplifier. This would overcome the effect of any remaining small difference of potential between the body and the bulk of the amplifier circuit (i.e. its 'common' or chassis connection). The combined result of these differences is that the number of body electrodes and the corresponding number of connecting cables is reduced from seven to three. In the general case, if n electrodes are applied to the body connected in pairs to separate patient worn amplifiers so that each amplifier connects to only two of the electrodes but individual electrodes may be shared between two or among more than two amplifiers then n electrodes can service nC2 = 0.5n(n-l) amplifiers. Ignoring the fact that some parameters may not be useful, this means 0.5n(n-l) different body parameters can be measured using n body electrodes and n connecting cables. The seven electrodes of Figure 1 could service 21 channels using the differential connecting arrangement analogous to that of Figure 2. In Figure 2, it will be seen that the first body electrode 6 is connected to the negative input of differential amplifier 5 and the negative input of differential amplifier 8. The body electrode 7 is connected to the positive input of differential amplifier 5 and the negative input of differential amplifier 10. The third body electrode is connected to the positive input of differential amplifier 10 and the positive input of differential amplifier 8. The outputs 11, 12 and 13 of differential amplifiers 5, 8 and 10 respectively are connected to data processing apparatus 14 which may simply be a recorder or a display or may combine these items alone or together with data processing apparatus in order to interpret the difference of potential arising at the pairs of body electrodes in terms of body parameters to be measured. The differential amplifiers, which are adapted to be carried by the subject, are preferably carried in a belt mounted pouch. The data processing apparatus can be similarly mounted, preferably in a casing for the amplifiers and apparatus.

Claims: CLAIMS 1. A body electrode monitor comprising a plurality of electrodes for sensing potentials at points of attachment on a living body, a plurality of amplifiers each having two signal inputs and adapted for attachment to the body, and data processing apparatus connected to the amplifiers, the sole connections between each amplifier and the electrodes being connections from the two signal inputs to two electrodes, different amplifiers being connected to different combinations of electrodes. 2. A monitor as claimed in claim 1 wherein a said electrode is connected to an input of a plurality of amplifiers. 3. A monitor as claimed in claim 2 wherein each electrode is connected to an input of a plurality of amplifiers. 4. A monitor as claimed in any one of claims 1 to 3 comprising a casing for the differential amplifiers and the data processing apparatus, the casing being adapted for carrying by the body. 5. A monitor as claimed in claim 4 wherein the casing is provided with belt attachment means. 6. A body electrode monitor substantially as herein described with reference to the accompanying drawings.

Inventors: NEILSON JAMES MCEWAN MCINTYRE, NEILSON, JAMES, MCEWAN, MCINTYRE
Applicants: NEILSON JAMES MCEWAN MCINTYRE, NEILSON, JAMES, MCEWAN, MCINTYRE
Application year: 1994
Main classification: A61B5
IPC codes: A61B5